The way I understand it (YMMV), Satan will be bound 1000 years. During this time, people will be born who will STILL have a choice to follow Christ or not. After the 1000 years, Satan will be released for a VERY short time to gather all those from the 1000-year reign of Christ who DO NOT want to follow God.
At that time, God sends fire from Heaven to consume Satan’s army, Satan himself is judged, and then he’s thrown into the Lake of Fire where the Anti-Christ and the False Prophet have already been for the previous 1000 years. (Rev. 20:7-10, if you want to look it up)
The old heaven and old Earth are destroyed, God creates a new Heaven and a new Earth, and we who follow Christ live for an eternity with Him in joy and peace.
Doesn’t God already know how all this is going to end and what people are going to do?